1. Field of the Invention
Example embodiments of the present invention relates in general to the field of an Organic Light Emitting Diode (OLED) display device having a pixel circuit and a driving method thereof, and more specifically to an Organic Light Emitting Diode (OLED) display device having a pixel circuit which use a thin film transistor (TFT) as an active device and a driving method thereof.
2. Description of the Related Art
Presently, the OLED display device as a thin-film type display device can apply a Passive Matrix (PM) driving method and hence an Active Matrix (AM) driving method, in the same method as the LCD in which has been used widely and commercially.
The passive matrix driving method can have a simple structure and apply data exactly to each pixel. However, the passive matrix driving method is difficult to be applied to a large screen and a high-precision display. Accordingly, the development of the active matrix driving method has been actively proceeding.
A pixel circuit of the OLED display device will be now explained with reference to FIGS. 1 and 2 according to a conventional active matrix driving method.
FIG. 1 is a schematic diagram illustrating the OLED display device having a pixel circuit according to a conventional active matrix method.
Referring to FIG. 1, in the OLED display device, a plurality of scan lines (X1, X2, X3, . . . , Xn) for selecting and non-selecting the pixels 30 for a desired scan cycle (e.g., a frame period according to a NTSC standard) and a plurality of data lines (Y1, Y2, Y3, . . . , Yn) for supplying luminance information so as to drive the pixels 30 are arranged in a matrix type. The pixels 30 are arranged in each intersection portion in which the scan lines and the data lines are arranged in the matrix type. The respective pixels 30 are composed of a pixel circuit.
The scan lines (X1, X2, X3, . . . , Xn) are connected to a scan line driving circuit 20, and the data lines (Y1, Y2, Y3, . . . , Yn) are connected to a data line driving circuit 10. A desired image can be represented by selecting sequentially the scan lines (X1, X2, X3, . . . , Xn) by the data line driving circuit 10, supplying a voltage (or current) of the luminance information from the data lines (Y1, Y2, Y3, . . . , Yn) by the data line driving circuit 10, and filling repeatedly the voltage of the luminance information.
In this case, the passive matrix type OLED display device emits light only while light-emitting elements included in the respective pixels 30 are being selected, while an active matrix type OLED display device continuously performs the light emission of the light-emitting elements even after the voltage filling of the luminance information is finished.
Thus, in the large screen and high-precision display, the active matrix type OLED display device is more superior to the passive matrix type OLED display device because the driving current level of the light-emitting element is low.
Hereinafter, a driving operation of the OLED display device having the plurality of pixels 30 will be explained in detailed.
First, the scan line driving circuit 20 selects one XN of the scan lines (X1, X2, X3, . . . , Xn) and transmits a selecting signal. In the data line driving circuit 10, the data of the luminance information is transmitted to pixels arranged in transverse direction via the data lines (Y1, Y2, Y3, . . . , Yn).
Then, the scan line driving circuit 20 transmits a non-selected signal to the selected scan line XN, and then selects the next scan line (XN+1) so as to transmit the select signal. If the selection signal and the non-selected signal are sequentially transmitted to the scan line, the OLED display device can obtain a desired display by transmitting repeatedly the data.
FIG. 2 is a circuit diagram illustrating a conventional pixel circuit according to an active matrix method.
Referring to FIG. 2, a pixel circuit for driving a pixel 30 includes two NMOS transistors T1 and T2, and an OLED. The pixel circuit includes the OLED, a first transistor T1 for controlling a current, a second transistor T2, and a capacitor CS.
A source terminal of the first transistor T1 is connected to a positive pole (i.e., anode), and a drain terminal thereof is connected to a positive power source (VDD). A gate terminal of the second transistor T2 is connected to a scan line XN, and a drain terminal thereof is connected to a data line YM, and a source terminal thereof is connected to the gate terminal of the first transistor T1 and the capacitor CST.
A negative pole (i.e., cathode) of the OLED is connected to a negative supply source (VSS). Thus, a current of the OLED is controlled by applying a voltage of the data line YM to the gate terminal of the first transistor T1 via the second transistor T2.
Hereinafter, a driving operation of the pixel circuit will be explained.
When the gate terminal of the second transistor T2 receives a selection signal from the scan line XN, the second transistor T2 is turned on. At this time, a voltage corresponding to luminance information, which is applied to the data line YM by the data line driving circuit, is transmitted to the gate terminal of the first transistor T1 via the second transistor T2, and the luminance information voltage is stored in the capacitor CST.
Accordingly, even while the second transistor T2 is turned off by receiving the non-selected signal supplied from the scan line XN over one frame period, the voltage of the gate terminal of the first transistor T1 is constantly hold by the capacitor CST and thus the current flowing to the OLED via the first transistor T1 is constantly maintained.
As such, in conventional pixel circuit, since the current flowing to the OLED is the same as the current flowing from the drain terminal of the first transistor T1 to the source terminal thereof, the current is controlled by the voltage of the gate terminal of the first transistor T1. However, the current may be different from the magnitude of a desired current due to a characteristic deterioration caused by operation of the first transistor T1 for a long time.
A thin film transistor used in the display device is an active element suitable for the large screen and high precision display. However, even though the thin film transistor is formed on the same substrate, there is a problem that a threshold voltage of the thin film transistor is increased by several hundreds of □ or more than 1 Volt according to time variation.
For example, even though a same signal potential Vw is supplied to the gate of the thin film transistor in different time (e.g., several month later), if the threshold voltage of the transistor included in each pixel is different, the current value flowing to the OLED deviates from a value necessary for each pixel, and thus long life span necessary for the display device can not be expected.
The increasing of the threshold voltage for varying the current flowing to the OLED can not avoid according as the time flows. The characteristic deterioration produced by utilizing the display device for a long time causes an initial value to be greatly varied and the deterioration of the OLED causes a luminance to be greatly varied. This enables the quality of definition or brightness of the display device to be greatly varied, thereby decreasing life span of the display device.
FIG. 3 is a circuit diagram illustrating a voltage compensating circuit according to a conventional active matrix method, and FIG. 4 is a timing chart illustrating a driving of the voltage compensating circuit according to the conventional active matrix method.
Referring to FIGS. 3 and 4, the conventional voltage compensating circuit usually compensates a threshold voltage of the first transistor T1, but does not compensate a field effect mobility of the first transistor T1 and a characteristic deterioration of the OLED.
FIG. 5 is a circuit diagram illustrating a current compensating pixel circuit and a timing chart for driving the circuit according to a conventional general active matrix method.
Referring to FIG. 5, the current compensating circuit compensates a current-voltage characteristic variance of the first transistor T1 and the OLED, but does not compensate an efficiency variance of the OLED.